1. Field of the Invention
The present invention relates to a wire electrical discharge machine that automatically corrects a machining route according to a corner angle.
2. Description of the Related Art
In wire electric discharge machining, it is known that a wire electrode is deflected in a direction different from (a direction opposite to or a direction making a predetermined angle with respect to) a machining advancing direction due to discharge repulsive force that occurs between the wire electrode and a workpiece, a turbulent flow of machining liquid, or the like. Although a wire electrode is deflected in a direction opposite to a machining advancing direction when a workpiece is machined into a linear shape by the wire electrode, the deflection does not affect the machined shape. However, the effect of deflection becomes apparent in machining of corner portions so that shape accuracy at corner portions is degraded too much, a so-called “corner shear drop” occurs, and a desired shape is not obtained.
FIG. 1 is a schematic diagram for describing machining of a corner portion using a wire electrode 3 of a wire electric discharge machine.
In wire electric discharge machining, in order for a workpiece 2 to be machined into a desired dimension, it is a common practice to create an offset route by adding the radius of the wire electrode 3 and a discharge gap to the dimension of an actual product shape and to move the wire electrode 3 along the created offset route. The offset route is referred to as a machining route 4. The value obtained by adding the radius of the wire electrode 3 and the discharge gap is referred to as an offset value.
As illustrated in FIG. 1, at a corner portion of the workpiece 2, the wire electrode 3 does not move along the machining route 4 due to deflection of the wire electrode 3 but moves along the inner side (see a wire electrode track 5 indicated by a broken line) of the machining route 4 so that the workpiece 2 is machined excessively. Due to this, a so-called “corner shear drop” (see a corner shear drop 6 resulting from deflection of the wire electrode) is formed, and a desired finished shape is not obtained. In order to deal with such a problem, the following two countermeasures are mainly proposed.
A first countermeasure alleviates deflection of a wire electrode by suppressing a machining speed or the amount of machining liquid, or extending a discharge pause time at corner portions (this countermeasure is also called “energy control”).
A second countermeasure corrects a machining route by taking deflection of a wire electrode into consideration.
FIG. 2 is a diagram for describing the second countermeasure that corrects a machining route in order to avoid a workpiece from being excessively machined due to deflection of a wire electrode.
When wire electric discharge machining is performed while correcting the machining route 4 of the wire electrode 3 according to a correction route 7, the wire electrode 3 moves along the wire electrode track 8. In this manner, it is possible to avoid the workpiece 2 from being excessively machined.
Among the first and second countermeasures described above, the second countermeasure (that corrects the machining route) has advantages in that the machining period of the wire electric discharge machine is shorter than the energy control method of the first countermeasure, and several machining route correction methods have been proposed. Among the proposed correction methods, a technique of correcting a machining route based on a deflection amount of a wire electrode is disclosed in Patent Documents below, for example.
Japanese Patent Application Laid-Open No. 61-219529 (JP 61-219529,A) discloses a wire electric discharge machine controller including a control unit that controls a relative movement amount of a wire electrode, a storage unit that stores a deflection amount of the wire electrode on a working surface, a computing unit that sequentially determines a machining direction of the wire electrode according to computation, and a driving unit that drives the wire electrode with the same correction amount as the deflection amount of the wire electrode.
Japanese Patent Application Laid-Open No. 7-285029 (JP 7-285029,A) discloses a sharp edge machining method including storing a deflection amount of a wire electrode under wire-cut discharge machining for each machining condition, excessively allowing the wire electrode to freely move in an advancing direction during punching, and correcting a machining route so as to make a cut during die machining.
Japanese Patent Application Laid-Open No. 7-24645 (JP 7-24645,A) discloses a wire electric discharge machining apparatus including a control unit that controls a relative movement amount of a wire electrode, a corner detecting unit that detects a corner portion within a machining route, and a machining route correcting unit that sequentially corrects a predetermined distance of a tangential movement, a predetermined distance of an extended movement, and a gradual returning movement at the detected corner portion.
Japanese Patent Application Laid-Open No. 11-207527 (JP 11-207527,A) discloses a machining route correction method that includes extending a first machining route at a corner portion in a machining advancing direction to be on a tangential line, setting second and third correction routes at angles larger than a machining corner angle, and correcting the machining route so as to return to an original machining route in a fourth route and also discloses a wire electric discharge machining apparatus for realizing the machining route correction method.
Although the techniques disclosed in JP 61-219529,A and JP 7-285029,A correct the machining route using the deflection amount of the wire electrode as a correction amount, these correction methods mainly focus on machining when the corner angle is 90° and do not discuss the other coverage areas in detail. It is widely known that, when the corner angle decreases (that is, 90° or smaller), the wire electrode is strongly affected by disturbance such as the flow of a machining liquid at a corner vertex portion and the machining accuracy at the corner portion is further degraded. Therefore, if the same correction amount is applied to all corner angles regardless of a corner angle, there is a problem that a desired machined shape is not always obtained.
The technique disclosed in JP 7-24645,A requires a plurality of correction amounts because the machining route is corrected in three steps or more. In this technique, a plurality of correction amounts are changed according to the corner angle, and the other correction amounts are determined based on an experimental value. Although some correction amounts are determined based on the corner angle, it is not described in detail how the correction amount is changed according to the corner angle. Moreover, since the other correction amounts are setting values and a large amount of time and effort is required to calculate various setting values that can deal with various machining patterns present in the world as needed by users, there is a problem in that it is practically very difficult to determine the correction amounts.
The technique disclosed in JP 11-207527,A requires a plurality of correction amounts because the machining route is corrected in a number of steps. The plurality of (four) correction amounts is determined based on a machining type or a thickness of a workpiece according to the corner angle. Even when the machining types are the same, the degree of deflection of a wire electrode is different depending on the material, the thickness, or the like of the workpiece, and desired shape accuracy is not always obtained if the machining route is corrected by the same correction amount. By the same reason, even when the thicknesses of workpieces are the same, if the machining types, the materials of the workpieces, or the like are different, there is a problem in that desired shape accuracy is not obtained unless the correction amount is adjusted. Moreover, JP 11-207527,A does not describe in detail how the correction amount is to be changed according to the corner angle.